Viscous Drag
Summary
- Overview
- Reduction Methods
- Laminar Flow Control (LFC)
- Compliant Walls
- Riblets
- Large-Eddy Breakup Devices (LEBU)
- Surface Additives (polymers, bubbles)
- Selected References
Viscous drag (or skin friction drag) is due to the stresses on the aerodynamic surfaces and in the boundary layer. The decreased momentum in the flowfield results in a corresponding loss of momentum of the aerodynamic system. Some of the physical aspects involved in the viscous drag loss are: presence of shear layers, turbulent transition, boundary layer separation.
The amount of energy losses depends largely on the aero- hydrodynamic system. On a
sailing boat it can range between 1/3 and (nearly) the total drag, depending on the
speed of the craft (at low speeds the viscous drag is large, in percent, whereas the
wave drag is low). Some typical viscous losses are listed below:
Methods for viscous drag reduction rely on techniques that alter the turbulence
structure and/or the wall characteristics. These methods are both powered (active
methods) and unpowered.
Active methods widely used include
The are also the passive methods, such as vortex generators, along with appropriate
design of the aerodynamic surfaces, by minimizing the wetted area and the
volume. Other sophisticated techniques include:
supersonic fighter 25-30 % large tranport aircraft 40 % executive aircraft 50 % VTOL aircraft 70-80 % underwater bodies 70 % ships at low/high speed 90-30 % gas pipelines 90 %
Reduction Methods
Selected References